This blog discusses current weather, weather prediction, climate issues, and other topics

Tuesday, March 21, 2017

A Good Winter for Northwest Potholes: Why?

This has been a big year for Northwest potholes, feared by drivers and bicyclists alike. And should not be a surprise, considering the weather has been "ideal" for producing potholes: more cold (sub-freezing) temperatures than the past few years and lots of precipitation.

There has been plenty of media coverage of the pothole epidemic (see below from KING5 and KIRO)

One well-covered pothole in Spokane took out 8 cars in a few minutes! In Spokane, the pothole problem became so crippling for cyclists and cars, that anarchists (Portland Anarchist Road Care) started filling them in! When anarchist are fixing roads instead of tearing stuff down, you know you have a problem.

Now, Seattle is well organized in the pothole business and the city even has a pothole map. The figure below shows the potholes that have are either pending to be fixed (blue) or repaired during the past 90 days. Road dangers lurk in much of the city. Be careful out there.

So why has this been such a bountiful year for axle-breaking, tire-destroying, potholes in our region?

Because it has been much colder than the past few years, with lots of sub-freezing weather and loads of precipitation.

Why is subfreezing weather important? To understand why you need to know how water density (and volume) changes with temperature. The figure below shows how the volume of a gram of water varies with temperature. Starting above freezing, as water cools down it reaches maximum density (minimum volume) around 4C, and then the volume increases slowly towards freezing. But then things get exciting: the volume increases substantially as water freezes. To put it another way, water expands substantially as it transitions from liquid water to ice.

Such expansion can produce tremendous force--cracking rocks or bursting pipes.

Water can get below a roadbed, either through small cracks or from the side. If temperatures drop below freezing, the water can expand, push up upon and breaking the roadway (see schematic below) When the temperature warms, the ice contracts leaving a void below the roadway, which can result in the road collapsing due to the weight of cars. The more times this process is repeated, the more the road can be undermined.

So below-freezing temperatures are helpful to produce potholes, as is plentiful precipitation to supply the water that freezes.

Let's take a look at temperatures this and previous winters. At Seattle (see below), lots of surface air temperatures (yellow line) that dropped below the normal lows (blue line) and freezing this winter. Much more that the previous two years.

At Spokane, it was an amazingly cold year, with several days dropping to 0F or below!

Portland had many days below freezing this winter, producing lots of ice and snow.

The figure below shows that all the urban areas were substantially wetter than normal, supplying lots of water to invade the sub-surface between the roadways

The bottom line of all this. A wet year with lots of moisture invaded roadway subsurfaces. With the coldest winter in years, with lots of subfreezing temperatures, there were excellent conditions for the production of tire-bursting potholes in Seattle, Portland and Spokane.

Seattle streets have been repeatedly patched and repatched, rather inexpertly in many cases. A SDOT crew comes out, dumps some asphalt mix into a pothole, tamps it down and calls it good. Very occasionally a section of street is repaved. Your tax dollars at work.